As shown in FIG. 6 and FIG. 7, a press brake c includes a lower beam e on which a die a having a V-shaped sectional contour is fixedly mounted and an upper beam d on which a punch b is fixedly mounted so that a plate f is bent by lowering the upper beam d. As the plate is bent in this way, the upper beam d is deflected upwardly and the lower beam e is deflected downwardly during the bending operation under the influence of a load exerted on the upper beam d and the lower beam e, as represented by one-dot chain lines in FIG. 7. As a result, there is caused a phenomenon of so-called intermediate opening that a bend angle .theta..sub.2 of the plate f becomes larger than a bend angle .theta..sub.1 at opposite ends of the plate f because of the aforementioned deflection (refer to FIG. 8).
To prevent the phenomenon of intermediate opening from being caused, a measure of crowning adjustment is taken such that the central part of the die as viewed in the longitudinal direction is previously bent upwardly as represented by a two-dot chain line in FIG. 7 to maintain a parallel relationship between the upper beam d and the lower beam during the bending operation. Such a crowning adjustment mechanism as mentioned above has been already known as an apparatus for eliminating a malfunction of so-called intermediate opening in a press brake wherein the apparatus is disclosed in an official gazette of Japanese Patent Publication NO. 47017/1985 of which patent application was filed by the common applicant to the present invention.
When the plate f is to be bent to a required bend angle by operating the press brake c, it is required that a so-called quantity of driving g, i.e., a distance between the upper end of the die a and the lower end of the punch b is correctly determined corresponding to the required bend angle. In view of the fact that the driving quantity g is definitely determined by presetting the lowermost position of the punch b, the press brake is equipped with a mechanism for adjusting a quantity of downward stroke of the upper beam d so that the downward stroke quantity of the upper beam d is set to an optimum value corresponding to the required bend angle with the aid of the foregoing adjusting mechanism. This kind of technology has been already known as an apparatus for controlling a bend angle in a press brake wherein the apparatus is disclosed in an official gazette of Japanese Patent Publication NO. 20927/1989.
According to the foregoing prior invention, working conditions associated with a required bend angle of a plate, a thickness of the plate and others are inputted into the apparatus so that a value of correction to be accomplished for the driving quantity is obtained based on the working conditions in additional consideration of mechanical characteristics of the press brake and elastic deformation characteristics of the plate and the corrected driving quantity is then obtained based on the corrected value of the driving quantity and the logical value derived from the driving quantity, whereby the lowermost position to be assumed by the punch is set corresponding to the corrected driving quantity.
With respect to the first-mentioned prior invention relating to a mechanism for adjusting a quantity of crowning, however, it has been found that the mechanism has the following drawback. Specifically, a quantity of crowning to be adjusted varies in correspondence to a quantity of deflection of each of the upper beam d and the lower beam e. In view of the fact that the deflection quantity varies depending on the working conditions associated with a bending operation, an operator usually performs trial bending operations by several times at every time when the working conditions vary, in order to determine a quantity of crowning to be adjusted for eliminating the malfunction of intermediate opening. After he performs an adjusting operation with reference to the thus determined quantity of crowning to be adjusted, he starts normal operations for successively bending a number of plates. However, the determination of the crowning adjustment quantity made on the basis of a try-and-error process imposes a requirement for high skillfulness on him. Other drawback is that a dimensional accuracy fluctuates from product to product depending on a degree of skilfullness owned by each operator. In addition, there is a possibility that the malfunction of intermediate fails to be eliminated. Another drawback is that an operational efficiency is substantially reduced by an extent equivalent to a period of time consumed for the trial bending operations.
On the other hand, with respect to the last-mentioned prior invention relating to an apparatus for controlling a bend angle associated with a bending operation to be performed by a press brake, the apparatus has an advantage that a required bend angle can automatically be obtained without any necessity for trial bending operations, since the lowermost position to be assumed by the punch is determined with the apparatus in additional consideration of mechanical characteristics of the press brake and elastic deformation characteristics of the plate. According to this prior invention, however, it has been found that the apparatus has a drawback that the bend angle practically obtained after completion of the operation for adjusting the crowning quantity deviates from the required bend angle, because variation of the bend angle caused by the operation performed for adjusting the crowning quantity is not taken into account.